This invention pertains to method for simulating the oxidative aging of asphalt binders and, more particularly, to an improved accelerated method for simulating an oxidative aging of an asphalt binder by subjecting the binder to pressurized air and elevated temperature produced by microwave radiation.
Asphalt is made from petroleum residues at refineries located throughout the United States. Before a production run of asphalt is released to the purchaser, such as a state highway department, a sample of the asphalt is first tested by the purchaser to assure that it meets certain predetermined specifications. One such test is to evaluate the relative resistance of the asphalt to oxidative aging over an extended period of in-service pavement use. This extended period is preferably five to 10 years of in-service aging in the field. Accordingly, a simulated aging of the asphalt sample must be performed.
A current standard practice for the accelerated aging (oxidation) of an asphalt binder using a pressurized aging vessel (xe2x80x9cPAVxe2x80x9d) is the American Association of State and Transportation Officials (xe2x80x9cAASHTOxe2x80x9d) provisional standard designated PP1, Edition 1A, dated September 1993. In this test, residue from the conduct of a Rolling Thin-Film Oven Test (xe2x80x9cRTFOTxe2x80x9d), ASTM Designation D2872-85, is used. This RTFOT procedure includes the preheating of an oven for a minimum of 16 hours. The resulting residue is then used in the above-identified PAV test which includes the conductive heating of the RTFOT produced residue for a 20-hour period in a pressurized vessel. During this testing period, the primary asphalt load awaits release at the plant. This resulting backup at the asphalt plant can result in storage problems which may require the asphalt supplier to build and store the production runs of asphalt in EPA-approved storage facilities. Such a procedure obviously results in increased costs for the manufacturer which are in turn passed on to the purchaser.
It is thus desirable to provide a new method for an accelerated aging (oxidation) of the asphalt binder sample which simulates the effects of the RTFOT and PAV tests on asphalt properties but in a much shorter time period. Heretofore it was believed that the heating of an asphalt sample beyond ambient pressure and 115xc2x0 C. would change the chemistry of the asphalt. Thus, any new method should not introduce any parameters that may affect the asphalt chemistry and the validity of the aging process.
In response thereto I have invented a new method for the accelerated aging of a neat asphalt binder which does not affect the chemistry of the asphalt. My process utilizes a programmable scientific microwave unit (1200 watts at 2450 MHz) capable of housing a series of six asphalt sample vessels on a rotatable turntable. The vessels, each containing a preheated 27xc2x0 C. sample of the asphalt load, are placed in the microwave unit and first xe2x80x9cramp heatedxe2x80x9d to increase the sample temperature from 27xc2x0 C. to 147xc2x0 C. at 3.08 MPa (440 psi) over a 60-minute period. Subsequently, the samples are microwave heated for three 60-minute periods at a relatively constant 147xc2x0 C. (xc2x11xc2x0 C.) at an air pressure of 440 psi. A final microwave 30-minute heating period occurs at a temperature of 147xc2x0 C. and 440 psi. During these five stages a built in microwave fan is operating at an 80% capacity with the turntable alternately rotating in clockwise and counterclockwise directions so as to evenly distribute the microwave radiation about the samples. The appropriate parameters of interest can then be measured as if the sample has undergone an oxidative aging which occurs in the asphalt binder both during the paving process as well as during five to 10 years of in-service use.
I have found that my method obtains in one procedure a residue that is equivalent to that residue obtained after using the combined xe2x80x9cRTFOT+PAVxe2x80x9d procedures. (See above note ASTM designation D2872-85, entitled xe2x80x9cStandard Test Method for the Effect of Heat and Air on a Moving Film of Asphalt (Rolling Thin-Film Oven Test)xe2x80x9d and AASHTO provisional standard entitled xe2x80x9cStandard Practice for Accelerated Aging of Asphalt Binder Using a Pressurization Vessel (PAV)xe2x80x9d of September 1993.)
Accordingly, it is a general object of my invention to provide an improved method for the accelerated oxidative aging of an asphalt binder by subjecting the sample to pressurized air at an elevated temperature as provided by means of microwave radiation.
Another general object of this invention is to provide in a one-step process an asphalt sample simulative of an oxidative aging that occurs in asphalt both during the paving process as well as during in-service use.
Still another particular object of the invention is to provide a method, as aforesaid, for providing an asphalt sample that is equivalent to samples obtained from the previously used xe2x80x9cRTFOT and PAVxe2x80x9d methods.
Another object of this invention is to provide an accelerated method, as aforesaid, which simulates the effects of previously known accelerated aging methods of without affecting the chemistry of the asphalt sample.
A further object of this invention is to provide a method, as aforesaid, wherein a higher heating temperature over a shorter period of time at a selected higher pressure is utilized than that used in previous methods while producing a comparable simulated aging of the asphalt sample.
Still another particular object of this invention is to provide a method, as aforesaid, which precludes the introduction of parameters into the asphalt sample which may affect the asphalt chemistry and deleteriously affect the oxidative aging process.
Other objects and advantages of this invention will become apparent from the following description taken in connection with the accompanying drawings, wherein is set forth by way of illustration and example, a now preferred method of this invention.